G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 1, Number 1 23 February , doi: /1999gc ISSN: The 40 Ar/ 39 Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic) Jörg Geldmacher, Paul van den Bogaard, Kaj Hoernle, and Hans-Ulrich Schmincke GEOMAR, Wischhofstrasse, 1-3D-24148, Kiel, Germany (jgeldmacher@ifm-geomar.de) [1] The 40 Ar/ 39 Ar ages for 35 volcanic rocks and 14 C ages for two charcoal samples from the Madeira Archipelago and Ampère Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (> Ma) and a subsequent low-volume posterosional stage (<0.7 0 Ma). Volcanism during the shield stage originated from a two-armed rift system, composed of the E W oriented Madeira rift arm and the N S oriented Desertas rift arm. Average growth rates for the submarine (5500 km 3 /Ma) and subaerial ( km 3 /Ma) shield stages on Madeira are among the lowest found for ocean island volcanoes. It is proposed that Madeira represents the present location of a >70 Myr old hotspot which formed Porto Santo Island ( Ma), Seine, Ampère (31 Ma), Corral Patch and Ormond (65 67 Ma [Féraud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70 72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at N/ W. Components: 9621 words, 10 figures, 4 tables. Keywords: Madeira; 40 Ar/ 39 Ar age dating; hotspot; Desertas Islands; Ampère Seamount; African plate motion. Index Terms: 1035 Geochronology; 8400 Volcanology; 8157 Tectonophysics: Plate motions: past (3040) Received 5 October 1999; Revised 22 December 1999; Accepted 22 December 1999; Published 23 February Geldmacher, J., P. van den Bogaard, K. Hoernle, and H.-U. Schmincke (2000), The 40 Ar/ 39 Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic), Geochem. Geophys. Geosyst., 1, 1008, doi: /1999gc Introduction [2] A long-standing question concerns the origin of the 1700 km belt of volcanism in the eastern North Atlantic located off the coast of Iberia and western Africa between 23 and 38 N (Figure 1)[see Schmincke, 1982]. The belt consists of three island groups (Canary, Selvagen, and Madeira Archipelagoes) and more than 20 large seamounts. Global seismic tomographic studies provide evidence for large-scale upwelling from depths of >500 km beneath the region [Zhang and Tanimoto, 1992; Hoernle et al., 1995], supporting a mantle plume origin as previously suggested [e.g., Morgan, 1972; Holik and Rabinowitz, 1991; Hoernle et al., 1991; Hoernle and Schmincke, 1993a, b]. [3] The Madeira Archipelago (Figure 2) is located near the southwestern termination of a broad alignment of scattered seamounts and volcanic ridges extending from the Iberian shelf almost 900 km to the southwest (Figure 1). On the basis of its spatial orientation, it has been proposed that this belt of volcanoes could represent a hotspot track [Morgan, Copyright 2000 by the American Geophysical Union 1 of 26

2 evolution of the Madeira/Desertas volcanic system. In this study we summarize the results of field studies and present 40 Ar/ 39 Ar age data from volcanic rocks from the Madeira Island group and Ampère Seamount. The results of geochemical studies will be presented elsewhere (J. Geldmacher and K. Hoernle, manuscript in preparation, 2000). 2. Geological Setting of Islands and Seamounts in the Madeira Chain Figure 1. Bathymetric map of seamounts and island groups in the eastern North Atlantic (only depth contours above 3500 m below sea level are shown). Source: TOPEX [Smith and Sandwell, 1997]. Azores- Gibraltar Fracture Zone after Verhbitsky and Zolotarev [1989]. Proposed Madeira hotspot track shown as thin dotted line (oldest available radiometric ages for each volcano are given in Ma; see text for details). 1981]. Morgan proposes that the trace of this hotspot extends into the Mesozoic, at which time the hotspot was located between Labrador and Greenland. The entire belt can be divided into a western complex of seamounts on the NE SW trending Madeira-Tore Rise and an eastern chain of large, isolated seamounts forming a slightly curved track toward Madeira Island, which we will henceforth refer to as the Madeira volcanic chain. Gravimetric studies of Josephine Seamount, located at the northern end of the Madeira-Tore rise, have shown that this seamount is in isostatic equilibrium with the underlying oceanic crust, leading to the interpretation that the entire rise structure originated on young lithosphere adjacent to the Mid- Atlantic Ridge [Peirce and Barton, 1991]. The eastern chain of isolated volcanoes includes Madeira with the small Desertas Islands, Porto Santo, Seine Seamount, possibly Unicorn Seamount, Ampère Seamount, Coral Patch Seamount, and Ormonde Seamount/Gorringe Bank. [4] We have undertaken volcanological, geochronological, and geochemical studies on the Madeira Island group and associated seamounts in order to elucidate the origin of the volcanism in the eastern North Atlantic and to constrain the magmatic 2.1. Madeira and Desertas Islands [5] The Madeira Archipelago consists of five principal islands: the main island of Madeira (728 km 2 ), the three narrow Desertas Islands (15 km 2 ) extending more than 60 km SSE of the eastern end of Madeira, and Porto Santo Island (69 km 2 )45km to the northeast of Madeira (Figure 2). The archipelago is located on 140 Myr old oceanic crust [Pitman and Talwani, 1972] and rises from more than 4000 m water depths up to 1862 m above sea level (summit of Pico Ruivo). Previous geological studies [Carvalho and Brandão, 1991], geological mapping [Zbyszewski et al., 1973, 1975], and palaeontological work (summarized by Mitchel- Thomé [1976]) have outlined a long and complex volcanic history. [6] Using lithostratigraphic criteria, the geology of Madeira can be divided into four units: Figure 2. Bathymetric map of the Madeira Archipelago. Source: TOPEX [Smith and Sandwell, 1997]. At the eastern tip of Madeira, the E W oriented Madeira Rift forms an angle of 110 with the narrower NNW SSE oriented Desertas rift arm. The directions of major dike swarms are shown schematically, as is the location of a postulated sector collapse fan. The seamount SW of Madeira extends to a height of 500 m below sea level and may represent the present location of the Madeira hotspot. 2of26

3 Geosystems G 3 geldmacher et al.: the 40 ar/ 39 ar age dating /1999GC [7] 1. The first is the submarine shield, about which little is known. [8] 2. The late Miocene to Pliocene basal unit consists primarily of volcanic breccias and pyroclastic deposits with minor lava flows (unit b1of Zbyszewski et al. [1975]). Locally, this unit is extensively cut by dikes which approach dike swarms in some regions, in particular in the central part of the island. [9] 3. The middle unit is composed primarily of Pliocene to Pleistocene alkalic basalt lava flows (units b 2 4 of Zbyszewski et al. [1975]). This unit covers most of the island (Figure 3) and forms lava sequences >500 m in thickness locally cut by dike swarms. [10] 4. The upper unit consists of scoria cones and intracanyon flows, which were for the most part erupted after considerable erosion of the island. [11] Published radiometric age data focus on the younger volcanic rocks yielding K/Ar ages ranging from 0.7 to 3 Ma [Watkins and Abdel-Monem, 1971; Ferreira et al., 1975; Féraud et al., 1981; Mata et al., 1995] except for two from lava flows with ages of 3.9 Ma [Ferreira et al., 1975] and 4.4 Ma (average of duplicate analyses from Mata et al. [1995]). Ages and stratigraphic relationships of the oldest rocks (>3 Ma) are poorly understood. No age or compositional data have been published from the Desertas Islands. [12] Both Madeira and the Desertas are characterized in their central regions by swarms of steeply dipping, partly sheeted dikes, normal faults and graben structures, and abundant cinder cones stacked one on another. These features are parallel to the long axes of Madeira (E W) and the Desertas Islands (NNW SSE) (Figure 2) and are thus characteristic of volcanic rift zones, along which the islands preferably grew by intrusion and extrusion as has been described for rift zones in the Hawaiian and Canary Islands [e.g., Walker, 1987; Carracedo, 1994]. As seen on a bathymetric map (Figure 2), the Desertas rift builds a 60 km long submarine ridge, rising from more than 4000 m water depths. The Madeira and Desertas rifts form an angle of 110 and intersect near the eastern tip of Madeira (São Lourenço peninsula). In summary, we consider the Madeira and Desertas ridges to form a single volcanic complex, consisting of an E W oriented Madeira rift arm and a NNW SSE oriented Desertas rift arm Porto Santo [13] Porto Santo is located 45 km to the northeast of Madeira; the two islands are separated by water depths of 2000 m. Porto Santo is lower in elevation (with its highest peak at 517 m) and more uniform in relief compared to Madeira. A large basaltic to trachytic, mainly submarine, clastic cone makes up the core of the NE part of the island (seamount stage) [Schmincke and Staudigel, 1976; R. Schmidt et al., manuscript in preparation, 2000]. Intercalated with the volcanic rocks are shallow water carbonates. The cone is dissected by voluminous trachytic and basaltic intrusions. A thick pile of submarine to subaerial alkali basaltic to hawaiitic lava flows and associated pillows forms the western part of the island. All units are cut by minor trachytic and basaltic intrusions and dikes, which form most hill tops and ridge crests on the island. Dikes show a preferred NE SW orientation in the western part of the island and a more radial arrangement in the east [Ferreira and Cotelo Neiva, 1997]. Published K/Ar ages of Porto Santo range from 12.3 to 13.1 Ma [Féraud et al., 1981] Seine, Unicorn, Ampère, and Coral Patch Seamounts [14] Seine Seamount is located 200 km NE of Porto Santo, rising from more than 4000 m to less than 200 m water depths. This round seamount has steep sides and a flat top characteristic of a guyot. Unicorn Seamount lies 100 km due north of Seine Seamount. Ampère and Coral Patch Seamounts are located 190 km NE of Seine Seamount. Bathymetric data show that the shape of Ampère Seamount is also similar to a guyot with a summit that extends to 59 m below sea level [Litvin et al., 1982; Marova and Yevsyukov, 1987]. Alkaline nepheline basaltoids have been described from two short drill holes on the top of the seamount [Matveyenkov et al., 1994]. The neighboring Coral Patch Seamount forms an elongated E W oriented structure rising up to 900 m below sea level Ormonde Seamount [15] The 250 km long Gorringe Bank, which lies along the Azores-Gibraltar fracture zone (the Eurasia-African Plate boundary), is dominated by two summits, the Gettysburg (west) and Ormonde (east) Seamounts, which almost reach sea level. Except for the Ormonde summit, the rest of Gorringe Bank consists primarily of altered tholeiitic basalt and serpentinized peridotite [Auzende et al., 3of26

4 Figure 3. Geological map of Madeira, Desertas, and Porto Santo based on Zbyszewski et al. [1975] and Ferreira and Cotelo Neiva [1997]. White circles mark sampling sites for age determination.numbers are in million years. Superscript numbers identify K/Ar dates from (1) Watkins and Abdel-Monem [1971], (2) Féraud et al. [1981], and (3) Mata et al. [1995]. From Mata et al. [1995], averages of replicate analyses are shown when replicates are within 2s error of each other. If not, the age with smallest error is used. Black triangles in the lower right-hand corner of boxes around the ages mark samples from dikes. Numbers with one asterisk are from 14 C radiocarbon ages of charcoal from this study and (with two asterisks) Schmincke [1998]. All other numbers without superscript indicators are 40 Ar/ 39 Ar age determinations from this study. 1978; Cyagor II Group, 1984; Matveyenkov et al., 1994] and is considered to be a fragment of oceanic lithosphere of Early Cretaceous age [Féraud et al., 1986] uplifted and slightly tilted along the Azores- Gibraltar fracture zone [Auzende et al., 1978]. In contrast, the younger volcanic rocks on top of Ormonde Seamount comprise a wide range of alkaline rocks, including alkali basalts, nephelinites, and phonolites [Cornen, 1982]. The 40 Ar/ 39 Ar age dating of Ormonde alkaline volcanic rocks yielded ages between 65 and 67 Ma [Féraud et al., 1982, 1986]. 3. Sample Description and Analytical Procedures [16] Samples from the Madeira archipelago (locations shown in Figure 3) were collected from all stratigraphic units (brief description and sampling sites are given in Table 1). From Madeira Island, 4of26

5 Table 1. Sample Description and Sampling Sites Sample Sample Type Location Latitude /Longitude Madeira Ma 85 basanitic lava flow northern exit of São Vicente village Ma 19 plag in pumice layer Street Encumeada-Paul da Serra, between first and second tunnel Ma 82c plag in trachyte intrusion below chapel of São Vicente Ma 146 plag in pumice layer behind church of São Martinho Ma 37 hawaiitic lava flow Street Encumeada-Paul da Serra, 1 km behind third tunnel Ma 107 plag in pumice layer lower part of road profile of Porto Novo valley (south side) Ma 75 alkali basaltic sill quarry near Encumeada pass Ma 152 alkali basaltic lava flow road cut east of Jardim do Mar (Rib. Funda valley, near road tunnel) Ma 44 plag in pumice layer Street Encumeada-paul da Serra, 0.5 km south of Casa Lombo do Mouro Ma 170 hawaiitic lava flow thin flow below lava pile at road cut south part of Machico bay Ma 227 transitional basalt boulder road cut at Cumeal village (Cural das Freiras valley) Ma 208 alkali basaltic lava flow lower part of eastern slope of Paul da Serra plateau Ma 203 plag in alkali basaltic flow lower part of eastern slope of Paul da Serra plateau transitional basalt dike rim Cruz da Guarda village (south of Porto da Cruz) transitional basalt dike rim Cruz da Guarda village (south of Porto da Cruz) Desertas Islands Ilhéu Chão K 22 alkali basaltic lava flow base of Ilhéu Chão lava flow sequence Deserta Grande DGR 9 basanitic dike wall behind big landslide fan near refuge B megacryst in scoria cone top plateau near southern end of the island DGR 47 basanitic lava flow Pedregal summit DGR 2 alkali basaltic lava flow base of lava flow sequence (south of refuge) K 31 alkali basalti lava flow base of lava flow sequence, shore just south of Ponta do Pedregal Ilhéu do Bugio K 15 alkali basaltic dyke shore near Canto do Furado K 5 plag in alkali basaltic dyke shore near Ponta da Estância K 11 alkali basaltic beach block shore near Canto do Furado of26

6 Table 1. (Continued) Sample Sample Type Location Latitude /Longitude Porto Santo K 43 basanitic intrusion near Pico Juliana summit K 38 plag in trachytic intrusion west slope of Pico Castelo (above airport) K 67a alkali basaltic dike ridge of Espigao summit K 48 alkali basaltic dike street between Serra de Dentro and Camacha (below small quarry west of Pico Branco) K 47 plag in trachytic intrusion street between Serra de Dentro and Camacha (small quarry west of Pico Branco) K 68 rachytic intrusion near ana Ferreira Summit K 42 plag in benmoreitic dike west slope of Pico Castelo (above airport) K 46 plag in basanitic intrusion below Pico Juliana summit K 55 alkali basaltic lava flow lower part of lava flow sequence at Zimbralinho bay (sw part of the island) K 49 plag in benmoreitic dike street between Serra de Dentro and Camacha (ne of Pico Juliana) Ampére Seamount DS hawaiitic beach cobble eastern part of summit at n/ w (160m water depth) five samples from the basal unit, eight samples from the middle unit, and two samples from the upper unit were dated. At the center of Madeira, a continuous section was sampled, extending from the basal unit through the middle and upper units on the eastern slope of Paul da Serra plateau (Figures 4 and 5). Desertas s samples comprise rocks from the upper and lower parts of the successions of all three islands. On Porto Santo, samples were taken from the NE and SW parts of the island and come from all stratigraphic units. [17] Rock samples from Madeira and Desertas islands chosen for age dating range from tholeiites to alkali basalt to basanite and hawaiite. Olivine is the dominant phenocryst phase. Ti-augite and/or plagioclase also occur as phenocrysts. The groundmass consists of plagioclase, Ti-augite, olivine, and Fe/Ti-oxides. Two glass samples from tholeiitic dike rims from the basal unit on Madeira were also dated. Six samples of plagioclase crystals were separated from thin trachytic pumice deposits and from one small rhyolitic intrusion. In contrast to Madeira and the Desertas, differentiated rocks are more common on Porto Santo. Plagioclase crystals from Porto Santo were separated from two benmoreitic dikes and two trachytic intrusions. Whole rock samples range from basanites to trachytes. Ampère Seamount sample DS is a rounded beach cobble of hawaiitic composition and was recovered during F.S. Poseidon cruise 235. Descriptions and locations of land and dredge samples are compiled in Table 1. [18] Following removal of altered surfaces, pieces of selected rock samples were crushed in a jaw crusher to <1 mm and <0.5 mm size and sieved. Matrix chips, fresh glass, or plagioclase phenocrysts were hand-picked under a binocular microscope and cleaned with distilled H 2 O in an ultrasonic disintegrator. Plagioclase crystals ( mm) were treated in 5% HF for 5 min. Plagioclase crystals and rock matrix chip samples were placed in drill holes in 99.95% pure aluminum disks. Sample disks with a three-dimensional array of Ma TCR (batch 85G003) sanidine monitor [Duffield and Dalrymple, 1990] were secured together, sealed in an aluminum can, and irradiated with 1 mm Cd shielding at the Geesthacht Research Center (Germany). J values and associated errors were interpolated for each sample position using a three-dimensional least squares cosine plane fit. The 40 Ar/ 39 Ar laser total fusion analyses were conducted at the Geomar Geochronology Laboratory using a 25 W Spectra Physics argon ion laser and a MAP 216 series mass 6of26

7 Figure 4. Stratigraphic sections are shown for Paul da Serra on (left) Madeira and (right) Deserta Grande summarizing the subaerial evolution of the Madeira/Desertas volcanic system. Elevations are given in meters above sea level (note that Paul da Serra profile is a composite section). All ages are in million years with 2s errors. The 14 C radiocarbon ages are calibrated to B.P. spectrometer fitted with a Baur-Signer ion source and a Johnson electron multiplier. Between 5 and 13 single grains (rock/glass fragments or plagioclase crystals) of each sample were completely fused (see Tables 2 and 3 for number of single particles analyzed). Raw mass spectrometer peaks were corrected for mass discrimination, background values (determined between every 4 or 5 analyses), and interfering neutron reactions on Ca and K using optical grade CaF 2 and K 2 SO 4 salts that had been irradiated together with the samples. Age uncertainties were calculated by partial differentiation of the age equation [Dalrymple and Duffield, 1988] and include uncertainties in the 7of26

8 Figure 5. Schematic outcrop sketch of Paul da Serra profile (eastern slope) in central Madeira. Encumeada pass is at the road junction at the right. All ages are in million years. determination of the flux monitor, J, the blank determination, the regression of the intensities of the individual isotopes, and the mass discrimination correction ( per AMU). Ages and error estimates were determined by calculating the mean apparent age of each population (single fusion ages weighted by the inverse of their variance following the method described by Young [1962]), assuming an initial atmospheric 40 Ar/ 39 Ar ratio of Isochrons have been calculated as inverse isochrons using York s [1969] least squares fit that accommodates errors in both ratios and correlation of errors. Mean squared weighted deviates (MSWD) were determined for the mean apparent ages and isochron ages in order to test the scatter of the single fusion data [Wendt and Carl, 1991]. If the scatter was greater than predicted from the analytical uncertainties (MSWD > 1), the analytical error has been expanded by multiplying by the square root of the MSWD [York, 1969]. Errors are quoted at the 2s level. Mean apparent ages, isochron ages, and mean square weighted deviates are reported in Table 1. Because of the reasonably good control of the initial 40 Ar/ 36 Ar ratios in isotope correlation diagrams (see Figures 6, 7, and 8), inherited or excess 40 Ar can be ruled out for samples within 2s error from the accepted initial value of or slightly lower. If the isotope correlation gives atmospheric initial Ar isotope ratios (within error), a relatively inaccurate determination of the initial ratio for the isochron age would tend to adulterate rather than to improve the absolute age. Therefore, except for sample K 48, the mean apparent age is accepted instead of the isochron age to represent the age of crystallization. Because sample K 48 has an elevated initial 40 Ar/ 36 Ar ratio (outside of 2s error from the accepted value), the isochron age is accepted for this sample instead of the apparent age. [19] Two samples of charcoal, at the base of a pyroclastic fallout deposit on top of Paul da Serra plateau on Madeira, were selected for radiocarbon dating (Table 4). Samples were leached in 1% HCL, 1% NaOH, and again with 1% HCL at 60 C. The graphitisized samples were analyzed for 14 C ratios with an accelerator mass spectrometer (AMS) at the Leibniz Laboratory at Christian Albrechts University in Kiel. The measured 14 C ratios are corrected for mass fractionation and converted into calibrated age after Stuiver and Reimerm [1993]. 4. Results 4.1. Madeira [20] The oldest dated rocks on Madeira occur in the vicinity of Porto da Cruz (NE Madeira, Figure 3). Fresh glass rims of tholeiitic dikes, cross-cutting volcanic breccia and pyroclastic rocks, gave 40 Ar/ 39 Ar ages of 4.48 ± 0.18 and 4.63 ± 0.10 Ma (samples and ). Lavas from the basal unit in Curral das Freiras valley and near the base of the eastern slope of Paul da Serra plateau (Figure 4) produced ages between 3.9 and 4.4 Ma (MA 227, MA 208, MA 203). Only one flow from Madeira dated in this study has an age between 2.8 and 3.9 Ma. Sample MA 170, with 8of26

9 Table 2. Analytical Results Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s MA107: J = ± MA E E 01 ± 1.01E ± E+06 ± 4.0E+05 MA E E 01 ± 1.29E ± E+06 ± 5.2E+04 MA E E 01 ± 3.48E ± E+06 ± 1.4E+05 MA E E 01 ± 2.16E ± E+06 ± 8.7E+04 MA E E 01 ± 3.63E ± E+06 ± 1.5E+05 MA E E 01 ± 6.12E ± E+06 ± 2.4E+05 MA E E 01 ± 4.43E ± E+06 ± 1.8E+05 MA E E 01 ± 5.81E ± E+05 ± 2.3E+05 MA E E 01 ± 2.26E ± E+06 ± 9.0E+04 MA E E 01 ± 6.63E ± E+06 ± 2.7E+05 MA E E 01 ± 3.89E ± E+06 ± 1.6E+05 MA E E 01 ± 6.05E ± E+06 ± 2.4E+05 MA E E 01 ± 6.01E ± E+06 ± 2.4E+05 MA146: J = ± MA E E 01 ± 1.21E ± E+06 ± 4.6E+05 MA E E 01 ± 2.76E ± E+06 ± 1.0E+06 MA E E 01 ± 1.57E ± E+06 ± 6.0E+05 MA E E 01 ± 1.45E ± E+06 ± 5.5E+05 MA E E 01 ± 1.50E ± E+06 ± 5.7E+05 MA E E 01 ± 3.31E ± E+06 ± 1.3E+06 MA E E 01 ± 5.72e ± E+06 ± 2.2E+05 MA E E 01 ± 2.61E ± E+06 ± 9.9E+05 MA E E 01 ± 2.52E ± E+06 ± 9.6E+05 MA E E 01 ± 1.77E ± E+06 ± 6.7E+05 MA19: J = ± MA E E 01 ± 1.04E ± E+06 ± 3.9E+05 MA E E 01 ± 6.68E ± E+06 ± 2.5E+05 MA E E 01 ± 5.28E ± E+06 ± 2.0E+05 MA E E 01 ± 4.86E ± E+06 ± 1.8E+05 MA E E 01 ± 6.92E ± E+06 ± 2.6E+05 MA E E 01 ± 7.06E ± E+06 ± 2.7E+05 MA E E 01 ± 5.27E ± E+06 ± 2.0E+05 MA E E 01 ± 4.31E ± E+06 ± 1.6E+05 MA E E 01 ± 9.88E ± E+06 ± 3.8E+05 MA E E 01 ± 5.36E ± E+06 ± 2.0E+05 MA E E 01 ± 1.54E ± E+06 ± 5.8E+05 MA44: J = ± MA E E 01 ± 7.72E ± E+06 ± 3.1E+05 MA E E 01 ± 3.89E ± E+06 ± 1.6E+05 MA E E 01 ± 2.54E ± E+06 ± 1.0E+05 MA E E 01 ± 1.63E ± E+06 ± 6.5E+04 MA E E 01 ± 3.54E ± E+06 ± 1.4E+05 MA E E 01 ± 4.52E ± E+06 ± 1.8E+05 MA E E 01 ± 1.46E ± E+06 ± 5.9E+04 MA E E 01 ± 1.62E ± E+06 ± 6.5E+04 MA E E 01 ± 2.84E ± E+06 ± 1.1E+05 MA E E 01 ± 1.64E ± E+06 ± 6.6E+04 MA E E 01 ± 3.92E ± E+06 ± 1.6E+05 MA82c: J = ± MA82c 2.54E E 01 ± 5.53E ± E+06 ± 2.1E+05 MA82c 6.18E E 01 ± 2.55E ± E+06 ± 9.7E+04 MA82c 5.74E E 01 ± 3.76E ± E+06 ± 1.4E+05 MA82c 8.14E E 01 ± 2.20E ± E+06 ± 8.4E+04 MA82c 4.75E E 01 ± 5.86E ± E+06 ± 2.2E+05 MA82c 4.94E E 01 ± 4.67E ± E+06 ± 1.8E+05 MA82c 3.14E E+00 ± 3.52E ± E+07 ± 1.3E+07 MA82c 4.42E E 01 ± 8.26E ± E+06 ± 3.1E+05 9of26

10 Table 2. (continued) Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s MA82c 2.02E E 01 ± 9.97E ± E+05 ± 3.8E+05 MA82c 2.19E E 01 ± 8.25E ± E+05 ± 3.1E : J = ± E E+01 ± 1.59E ± E+07 ± 3.0E E E+01 ± 1.85E ± E+07 ± 3.5E E E+01 ± 1.68E ± E+07 ± 3.2E E E+01 ± 1.28E ± E+07 ± 2.5E E E+01 ± 1.08E ± E+07 ± 2.1E E E+01 ± 8.08E ± E+07 ± 1.7E E E+01 ± 1.47E ± E+07 ± 2.8E E E+01 ± 1.46E ± E+07 ± 2.8E E E+01 ± 3.77E ± E+07 ± 7.0E E E+01 ± 4.43E ± E+07 ± 8.2E E E+01 ± 3.00E ± E+07 ± 5.6E E E+01 ± 3.29E ± E+07 ± 6.1E E E+01 ± 2.31E ± E+07 ± 4.3E+05 K42: J = ± K E E+00 ± 5.69E ± E+07 ± 1.1E+06 K E E+00 ± 3.62E ± E+07 ± 6.7E+05 K E E+00 ± 1.19E ± E+07 ± 2.2E+06 K E E+00 ± 1.97E ± E+07 ± 3.6E+06 K E E+00 ± 4.65E ± E+07 ± 8.6E+05 K E E+00 ± 1.11E ± E+07 ± 2.0E+06 K E E+00 ± 4.96E ± E+07 ± 9.2E+05 K E E+00 ± 9.03E ± E+07 ± 1.7E+06 K E E+00 ± 2.29E ± E+07 ± 4.2E+05 K E E+00 ± 4.08E ± E+07 ± 7.5E+05 K E E+00 ± 2.58E ± E+07 ± 4.8E+05 K46: J = ± K E E+01 ± 2.29E ± E+07 ± 4.2E+06 K E E+00 ± 1.16E ± E+07 ± 2.1E+06 K E E+00 ± 3.58E ± E+07 ± 6.6E+05 K E E+00 ± 3.30E ± E+07 ± 6.1E+05 K E E+00 ± 3.97E ± E+07 ± 7.4E+05 K E E+00 ± 3.22E ± E+07 ± 6.0E+05 K E E+00 ± 4.65E ± E+07 ± 8.6E+05 K E E+00 ± 2.42E ± E+07 ± 4.5E+05 K E E+00 ± 7.17E ± E+07 ± 1.3E+06 K E E+00 ± 1.95E ± E+07 ± 3.6E+06 K49: J = ± K E E+00 ± 3.06E ± E+07 ± 5.6E+05 k E E+00 ± 6.29E ± E+07 ± 1.2E+06 K E E+00 ± 1.48E ± E+07 ± 2.8E+05 K E E+00 ± 2.36E ± E+07 ± 4.4E+05 k E E+00 ± 5.91E ± E+07 ± 1.1E+06 K E E+00 ± 1.11E ± E+07 ± 2.1E+05 k E E+00 ± 9.73E ± E+07 ± 1.8E+06 K E E+00 ± 4.77E ± E+07 ± 8.8E+05 K E E+00 ± 5.85E ± E+07 ± 1.1E+06 K E E+00 ± 1.41E ± E+07 ± 2.6E+05 K E E+00 ± 1.27E ± E+07 ± 2.4E+05 K55: J = ± K E E+00 ± 2.51E ± E+07 ± 4.7E+05 K E E+00 ± 1.23E ± E+07 ± 2.3E+05 K E E+00 ± 2.45E ± E+07 ± 4.5E+05 K E E+00 ± 2.38E ± E+07 ± 4.4E+05 K E E+00 ± 7.65E ± E+07 ± 1.4E of 26

11 Table 2. (continued) Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s K E E+00 ± 1.53E ± E+07 ± 2.8E+05 K E E+00 ± 1.70E ± E+07 ± 3.2E+05 K48: J = ± K E E+00 ± 2.97E ± E+07 ± 5.5E+05 K E E+00 ± 3.15E ± E+07 ± 5.8E+05 K E E+00 ± 1.71E ± E+07 ± 3.2E+05 K E E+00 ± 8.10E ± E+07 ± 1.5E+05 K E E+00 ± 1.35E ± E+07 ± 2.5E+05 K E E+00 ± 8.99E ± E+07 ± 1.7E+05 K47: J = ± K E E+00 ± 4.38E ± E+07 ± 8.1E+05 K E E+00 ± 3.13E ± E+07 ± 5.8E+05 K E E+00 ± 5.55E ± E+07 ± 1.0E+06 K E E+00 ± 3.72E ± E+07 ± 6.9E+05 K E E+00 ± 8.73E ± E+07 ± 1.6E+06 K E E+00 ± 1.36E ± E+07 ± 2.5E+05 K E E+00 ± 2.17E ± E+07 ± 4.0E+05 K E E+00 ± 2.19E ± E+07 ± 4.1E+05 K E E+00 ± 2.98E ± E+07 ± 5.5E+05 K E E+00 ± 1.54E ± E+07 ± 2.9E+05 K E E+00 ± 1.58E ± E+07 ± 2.9E+05 K67a: J = ± K67a 2.56E E+00 ± 4.16E ± E+07 ± 7.7E+05 K67a 6.61E E+00 ± 9.07E ± E+07 ± 1.7E+05 K67a 3.75E E+00 ± 1.84E ± E+07 ± 3.4E+05 K67a 5.72E E+00 ± 5.89E ± E+07 ± 1.1E+05 K67a 4.77E E+00 ± 1.22E ± E+07 ± 2.3E+05 K67a 7.46E E+00 ± 4.71E ± E+07 ± 9.1E+04 K67a 9.13E E+00 ± 1.26E ± E+07 ± 2.3E+05 K67a 4.10E E+00 ± 1.35E ± E+07 ± 2.5E+05 K68: J = ± K E E+00 ± 7.45E ± E+07 ± 1.4E+05 K E E+00 ± 9.12E ± E+07 ± 1.7E+05 K E E+00 ± 7.87E ± E+07 ± 1.5E+05 K E E+00 ± 4.27E ± E+07 ± 8.3E+04 K E E+00 ± 6.48E ± E+07 ± 1.2E+05 K E E+00 ± 5.90E ± E+07 ± 1.1E+05 K E E+00 ± 4.24E ± E+07 ± 8.2E : J = ± E E+00 ± 2.45E ± E+06 ± 4.6E E E+00 ± 1.17E ± E+06 ± 2.2E E E+00 ± 8.40E ± E+06 ± 1.6E E E+00 ± 9.39E ± E+06 ± 1.8E E E+00 ± 1.03E ± E+06 ± 1.9E E E+00 ± 9.95E ± E+06 ± 1.9E E E+00 ± 1.87E ± E+06 ± 3.5E : J = ± E E+00 ± 5.46E ± E+06 ± 1.0E E E+00 ± 9.44E ± E+06 ± 1.8E E E+00 ± 1.02E ± E+06 ± 1.9E E E+00 ± 1.06E ± E+06 ± 2.0E E E+00 ± 1.60E ± E+06 ± 3.0E E E+00 ± 3.89E ± E+06 ± 7.3E of 26

12 Table 2. (continued) Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s 6302B: J = ± B 3.54E E+00 ± 1.16E ± E+06 ± 2.2E B 2.14E E+00 ± 1.62E ± E+06 ± 3.0E B 9.21E E+00 ± 5.97E ± E+06 ± 1.1E B 8.90E E+00 ± 4.81E ± E+06 ± 9.0E B 9.09E E+00 ± 5.76E ± E+06 ± 1.1E B 7.50E E+00 ± 6.66E ± E+06 ± 1.2E B 3.92E E+00 ± 8.36E ± E+06 ± 1.6E B 3.48E E+00 ± 1.14E ± E+06 ± 2.1E B 1.55E E+00 ± 2.66E ± E+06 ± 5.0E B 4.34E E+00 ± 4.35E ± E+06 ± 8.1E+04 DGR2: J = ± DGR2 2.24E E+00 ± 3.45E ± E+06 ± 6.4E+05 DGR2 1.20E E+00 ± 5.73E ± E+06 ± 1.1E+05 DGR2 4.81E E+00 ± 1.53E ± E+06 ± 2.9E+05 DGR2 3.08E E+00 ± 2.16E ± E+06 ± 4.0E+05 DGR2 6.73E E+00 ± 1.37E ± E+06 ± 2.6E+05 DGR2 9.01E E+00 ± 1.91E ± E+06 ± 3.6E+05 DGR2 1.15E E+00 ± 9.89E ± E+06 ± 1.8E+05 DGR47: J = ± DGR E E+00 ± 1.45E ± E+06 ± 2.7E+05 DGR E E+00 ± 6.13E ± E+06 ± 1.1E+05 DGR E E+00 ± 6.03E ± E+06 ± 1.1E+05 DGR E E+00 ± 6.85E ± E+06 ± 1.3E+05 DGR E E+00 ± 1.43E ± E+06 ± 2.7E+05 DGR E E+00 ± 8.65E ± E+06 ± 1.6E+05 DGR9: J = ± DGR9 6.55E E+00 ± 1.18E ± E+06 ± 2.2E+05 DGR9 2.21E E+00 ± 2.03E ± E+06 ± 3.9E+04 DGR9 1.55E E+00 ± 4.91E ± E+06 ± 9.2E+04 DGR9 1.04E E+00 ± 6.10E ± E+06 ± 1.1E+05 DGR9 8.48E E+00 ± 6.83E ± E+06 ± 1.3E+05 DGR9 3.85E E+00 ± 1.21E ± E+06 ± 2.3E+05 K11: J = ± K E E+00 ± 1.37E ± E+06 ± 2.5E+05 K E E+00 ± 2.82E ± E+06 ± 5.3E+04 K E E+00 ± 4.54E ± E+06 ± 8.5E+04 K E E+00 ± 7.08E ± E+06 ± 1.3E+05 K E E+00 ± 5.89E ± E+06 ± 1.1E+05 K E E+00 ± 2.74E ± E+06 ± 5.2E+04 K E E+00 ± 7.00E ± E+06 ± 1.3E+05 K15: J = ± K E E+00 ± 1.93E ± E+06 ± 3.7E+04 K E E+00 ± 6.38E ± E+06 ± 1.2E+05 K E E+00 ± 2.27E ± E+06 ± 4.3E+04 K E E+00 ± 8.05E ± E+06 ± 1.5E+05 K E E+00 ± 5.88E ± E+06 ± 1.1E+05 K22: J = ± K E E+00 ± 2.08E ± E+06 ± 3.9E+05 K E E+00 ± 1.10E ± E+06 ± 2.1E+05 K E E+00 ± 1.81E ± E+06 ± 3.4E+05 K E E+00 ± 2.54E ± E+06 ± 4.7E+05 K E E+00 ± 2.20E ± E+06 ± 4.1E+05 K E E+00 ± 4.06E ± E+06 ± 7.6E+05 K E E+00 ± 2.33E ± E+06 ± 4.3E+05 K E E+00 ± 3.90E ± E+06 ± 7.3E of 26

13 Table 2. (continued) Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s K E E+00 ± 1.65E ± E+06 ± 3.1E+05 K E E+00 ± 2.20E ± E+06 ± 4.1E+05 K31: J = ± K E E+00 ± 4.90E ± E+06 ± 9.2E+04 K E E+00 ± 5.33E ± E+06 ± 1.0E+05 K E E+00 ± 8.84E ± E+06 ± 1.7E+05 K E E+00 ± 6.49E ± E+06 ± 1.2E+05 K E E+00 ± 5.99E ± E+06 ± 1.1E+05 K38: J = ± K E E+00 ± 7.38E ± E+07 ± 1.4E+06 K E E+00 ± 9.15E ± E+07 ± 1.7E+06 K E E+00 ± 3.35E ± E+07 ± 6.2E+05 K E E+00 ± 3.19E ± E+07 ± 5.9E+05 K E E+00 ± 1.50E ± E+07 ± 2.8E+05 K E E+00 ± 2.19E ± E+07 ± 4.0E+05 K E E+00 ± 2.78E ± E+07 ± 5.1E+05 K E E+00 ± 2.54E ± E+07 ± 4.7E+05 K E E+00 ± 2.81E ± E+07 ± 5.2E+05 K E E+00 ± 2.19E ± E+07 ± 4.1E+05 K E E+00 ± 2.05E ± E+07 ± 3.8E+05 K43: J = ± K E E+00 ± 8.92E ± E+07 ± 1.7E+05 K E E+00 ± 6.79E ± E+07 ± 1.3E+05 K E E+00 ± 9.48E ± E+07 ± 1.8E+05 K E E+00 ± 6.59E ± E+07 ± 1.2E+05 K E E+00 ± 6.10E ± E+07 ± 1.1E+05 K E E+00 ± 4.25E ± E+07 ± 8.2E+04 K E E+00 ± 4.47E ± E+07 ± 8.6E+04 K E E+00 ± 7.20E ± E+07 ± 1.3E+05 K5: J = ± K5 3.77E E+00 ± 3.87E ± E+06 ± 7.2E+05 K5 4.90E E+00 ± 5.50E ± E+06 ± 1.0E+06 K5 1.97E E+00 ± 5.95E ± E+06 ± 1.1E+05 K5 7.68E E+00 ± 3.04E ± E+06 ± 5.7E+05 K5 4.84E E+00 ± 4.37E ± E+06 ± 8.2E+05 K5 3.13E E+00 ± 3.41E ± E+06 ± 6.4E+05 K5 4.84E E+00 ± 2.08E ± E+06 ± 3.9E+05 K5 1.13E E+00 ± 2.19E ± E+06 ± 4.1E+05 K5 1.11E E+00 ± 1.97E ± E+06 ± 3.7E+05 MA152: J = ± MA E E+00 ± 3.25E ± E+06 ± 6.0E+05 MA E E+00 ± 7.96E ± E+06 ± 1.5E+05 MA E E+00 ± 4.41E ± E+06 ± 8.2E+04 MA E E+00 ± 4.79E ± E+06 ± 8.9E+04 MA E E+00 ± 5.75E ± E+06 ± 1.1E+05 MA E E+00 ± 3.21E ± E+06 ± 6.0E+04 MA170: J = ± MA E E+00 ± 1.13E ± E+06 ± 2.1E+05 MA E E+00 ± 9.78E ± E+06 ± 1.8E+05 MA E E+00 ± 6.43E ± E+06 ± 1.2E+05 MA E E+00 ± 4.43E ± E+06 ± 8.3E+04 MA E E+00 ± 5.34E ± E+06 ± 1.0E+05 MA E E+00 ± 5.27E ± E+06 ± 9.8E+04 MA E E+00 ± 3.94E ± E+06 ± 7.4E+04 MA203: J = ± of 26

14 Table 2. (continued) Sample Mass, mg 40* 39 K 1s 40 Ar atm % CA/K 1s Apparent Age 1s MA E E+00 ± 1.37E ± E+06 ± 2.6E+06 MA E E+00 ± 1.30E ± E+06 ± 2.4E+06 MA E E+00 ± 2.63E ± E+06 ± 4.9E+05 MA E E+00 ± 9.86E ± E+06 ± 1.8E+06 MA E E 01 ± 1.40E ± E+06 ± 2.6E+06 MA E E 01 ± 1.15E ± E+05 ± 2.1E+06 MA E E+00 ± 9.53E ± E+06 ± 1.8E+06 MA208: J = ± MA E E+00 ± 4.14E ± E+06 ± 7.7E+04 MA E E+00 ± 6.32E ± E+06 ± 1.2E+05 MA E E+00 ± 3.75E ± E+06 ± 7.0E+04 MA E E+00 ± 3.49E ± E+06 ± 6.5E+04 MA E E+00 ± 4.45E ± E+06 ± 8.3E+04 MA E E+00 ± 1.11E ± E+06 ± 2.1E+05 MA227: J = ± MA E E+00 ± 1.18E ± E+06 ± 2.2E+05 MA E E+00 ± 9.79E ± E+06 ± 1.8E+05 MA E E+00 ± 8.47E ± E+06 ± 1.6E+05 MA E E+00 ± 6.99E ± E+06 ± 1.3E+05 MA E E+00 ± 5.48E ± E+06 ± 1.0E+05 MA E E+00 ± 5.45E ± E+06 ± 1.0E+05 MA E E+00 ± 4.02E ± E+06 ± 7.6E+04 MA37: J = ± MA E E+00 ± 2.11E ± E+06 ± 3.9E+05 MA E E 01 ± 1.17E ± E+06 ± 2.2E+05 MA E E 01 ± 6.26E ± E+06 ± 1.2E+05 MA E E 01 ± 7.78E ± E+06 ± 1.5E+05 MA E E 01 ± 3.21E ± E+06 ± 6.0E+04 MA E E 01 ± 7.96E ± E+06 ± 1.5E+05 MA E E 01 ± 5.46E ± E+06 ± 1.0E+05 MA E E 01 ± 2.35E ± E+06 ± 4.4E+04 MA E E 01 ± 4.83E ± E+06 ± 9.0E+04 MA E E 01 ± 2.74E ± E+06 ± 5.1E+04 MA85: J = ± MA E E 02 ± 3.37E ± E+05 ± 6.3E+04 MA E E 02 ± 4.60E ± E+05 ± 8.6E+04 MA E E 01 ± 8.38E ± E+05 ± 1.6E+05 MA E E 02 ± 3.33E ± E+04 ± 6.2E+04 MA E E 02 ± 5.39E ± E+05 ± 1.0E+05 MA E E 02 ± 5.62E ± E+04 ± 1.1E+05 MA E E 01 ± 3.32E ± E+05 ± 6.2E+04 MA E E 01 ± 4.79E ± E+05 ± 9.0E+04 MA E E 02 ± 3.87E ± E+04 ± 7.2E+04 MA E E 02 ± 7.62E ± E+04 ± 1.4E+05 MA75: J = ± MA E E+00 ± 3.59E ± E+06 ± 6.7E+05 MA E E 01 ± 1.90E ± E+06 ± 3.6E+05 MA E E+00 ± 1.28E ± E+06 ± 2.4E+05 MA E E 01 ± 1.77E ± E+06 ± 3.3E+05 MA E E 01 ± 2.47E ± E+06 ± 4.6E+05 MA E E 01 ± 8.30E ± E+06 ± 1.5E of 26

15 Table 3. Radiometric Ages Calculated From 40 Ar/ 39 Ar Isotope Composition of Whole Rocks, Plagioclase, and Amphibole Phenocrysts Sample N Mean Apparent Age, Ma MSWD Isochron Age, Ma MSWD (isochron) Initial 40 AR/ 36 AR Madeira MA 85 (wr) ± 0.08* ± ± 16 MA 19 (plag) ± 0.16* ± ± 38 MA 82c (plag) ± 0.10* ± ± 19 MA 146 (plag) ± 0.30* ± ± 34 MA 37 (wr) ± 0.08* ± ± 28 MA 107 (plag) ± 0.06* ± ± 4 MA 75 (wr) ± 0.36* ± ± 7 MA 152 (wr) ± 0.10* ± ± 9 MA 44 (plag) ± 0.06* ± ± 2 MA 170 (wr) ± 0.10* ± ± 71 MA 227 (wr) ± 0.08* ± ± 4 MA 208 (wr) ± 0.06* ± ± 20 MA 203 (plag) ± 0.40* ± ± (glass) ± 0.18* ± ± (glass) ± 0.10* ± ± 4 Desertas Islands Ilhéu Chão K 22 (wr) ± 0.24* ± ± 50 Deserta Grande DGR 9 (wr) ± 0.08* ± ± B (amph) ± 0.12* ± ± 82 DGR 47 (wr) ± 0.12* ± ± 40 DGR 2 (wr) ± 0.18* ± ± 6 K 31 (wr) ± 0.14* ± ± 74 Ilhéu do Bugio K 15 (wr) ± 0.94* ± ± 19 K 5 (plag) ± 0.2* ± ± 31 K 11 (wr) ± 0.06* ± ± 24 Porto Santo K 43 (wr) ± 0.10* ± ± 20 K 38 (plag) ± 0.30* ± ± 4 K 67a (wr) ± 0.12* ± ± 9 K 48 (wr) ± ± 0.82* ± 4 K 47 (plag) ± 0.26* ± ± 19 K 68 (wr) ± 0.08* ± ± 82 K 42 (plag) ± 0.58* ± ± 9 K 46 (plag) ± 0.58* ± ± 12 K 55 (wr) ± 0.34* ± ± 30 K 49 (plag) ± 0.22* ± ± 4 Ampére Seamount DS (wr) ± 0.20* ± ± 6 *Accepted ages. N, number of single crystals/whole rock particles fused. Errors are quoted at the 2s level. MSWD, mean square weighted deviates; wr, whole rock; plag, plagioclase; amph, amphibole phenocrysts. an age of 3.42 ± 0.10 Ma, was taken from a lava flow at Machico near the easternmost tip of Madeira, where Madeira joins the Desertas submarine ridge (see Figure 2). [21] Samples from the middle unit range in age from 1.0 to 2.8 Ma. At Paul da Serra (Figure 5), the middle unit fills in a paleocanyon in the basal unit, reaching a thickness of up to 500 m. Plagioclase crystals (MA 44) from a pumice layer, just above 15 of 26

16 Figure 6. Isotope correlation diagrams for Ar isotope compositions of fused plagioclase phenocrysts, glass, and whole rock samples of Madeira. this unconformity on the paleohill forming part of the southern wall of the canyon, yielded an age of 2.80 ± 0.06 Ma. The unconformity separates this pumice layer from basal unit samples MA 208 and MA 203 with ages of 4.10 ± 0.06 and 4.36 ± 0.40 Ma sampled about 300 m below the unconformity at the same locality. A lava flow (MA 37) and pumice layer (MA 19), stratigraphically overlying the 2.8 Myr old pumice layer, were dated at 1.42 ± 0.08 and 1.05 ± 0.16 Ma, respectively. Additional samples from the middle unit (MA 75, MA 152, MA 146, and MA 107) fall within the range found at Paul da Serra ( Ma). In the southeastern part of the island (at the coast halfway between Funchal and Machico), Watkins and Abdel-Monem [1971] report K/Ar ages between 1.76 Ma at the base and 0.74 Ma at the top of a road profile (Figure 3). Plagioclase crystals (MA 107) from a pumice layer between the second (1.64 Ma) and the third (1.05 Ma) lowest flow produced an 16 of 26

17 Figure 7. Isotope correlation diagrams for Ar isotope compositions of fused plagioclase phenocryst and whole rock samples from Porto Santo. 40 Ar/ 39 Ar age of 1.44 ± 0.06 Ma, in good agreement with the K/Ar ages of Watkins and Abdel- Monem [1971]. [22] The upper unit on Madeira consists of numerous cinder cones and intracanyon lava flows, for example, at Porto do Moniz, Seixal, and São Vicente (Figure 3). Sample MA 85 from the pahoehoe lava sequence at São Vicente, which originates from the top of Paul da Serra, yields a young age of 0.18 ± 0.08 Ma. Two charcoal samples were collected beneath a tephra layer on top of Paul da Serra. The samples have calibrated 14 C ages of years B.P. (KIA 685) and years B.P. (KIA 686), confirming a Holocene age of 6450 years B.P., as previously mentioned by Schmincke [1998] Desertas Islands (Ilhéu Chão, Deserta Grande, and Ilhéu do Bugio) [23] All nine rocks dated from the Desertas Islands fall within a narrow range of Ma (Figure 3). A profile through the entire stratigraphic column exposed on Deserta Grande is shown in Figure 4. The oldest ages come from the lava flows at the base of the lava succession on Deserta Grande and Ilhéu Chão with 3.62 ± 0.14 (K 31), 3.48 ± 0.18 Ma (DGR 2), and 3.62 ± 0.24 (K 22), respectively. Samples from a lava flow (DGR 47) and from an amphibole megacryst in a scoria cone from the top of Deserta Grande yielded identical ages within error of 3.38 ± 0.12 and 3.36 ± 0.12, respectively. A young dike (DGR 9) that cuts the entire lava pile gives a similar age of 3.25 ± 0.08 Ma. A basaltic beach cobble from the western 17 of 26

18 Figure 8. Isotope correlation diagrams for Ar isotope compositions of fused whole rock and plagioclase phenocryst samples from the Desertas Islands and Ampère Seamount. shore of Ilhéu do Bugio produced an age of 3.35 ± 0.06 Ma (K 11); dikes from the same island which cut the entire lava sequence give ages of 3.35 ± 0.06 and 3.36 ± 0.2 Ma (K5, K15), within error of the youngest units from Deserta Grande Porto Santo [24] Samples from Porto Santo have older ages ( Ma) than found on Madeira or the Desertas Islands (Table 3, Figure 3). The investigated samples cover the entire stratigraphic succession of the island: clastic seamount stage in the NE, alkali basaltic lava sequence in the SW, and dikes and minor trachytic and basaltic intrusions which cut both units. The oldest age (14.31 ± 0.22 Ma) comes from a benmoreitic dike (K 49) associated with a trachytic intrusion belonging to the seamount stage in the northwest part of the island. Sample K 55 from a submarine lava flow with an age of ± 0.34 Ma comes from the base of the basaltic to hawaiitic sequence of lava flows at the southwestern end of the island. Dikes and intrusions of both mafic and evolved composition (K 38, K 42, K 47, K 67a, K 68) from the NE and SW of the island yield ages between 12.5 and 13.2 Ma (see Table 3). The youngest age from Porto Santo (11.07 ± 0.10 Ma) comes from a basanitic intrusion (K 43) Ampère Seamount [25] A rounded beach cobble of hawaiitic composition (DS 797-1) from Ampère Seamount gives the oldest age (31.2 ± 0.2 Ma) of the samples dated in this study. This sample was dredged from the Table 4. Sample Radiocarbon Ages of Charcoal Conventional Age, years B.P. Corrected FMC d 13 C, % Calibrated Age, * years B.P. KIA ± ± ± KIA ± ± ± * Translation into calibrated age according to Stuiver and Reimer[1993]. Ages referred to before present (1950). Errors at 1s level. 18 of 26

19 eastern part of the summit plateau of Ampère in 160 m water depth (see Table 1). 5. Discussion 5.1. Geochronological Evolution of Madeira and the Desertas Islands [26] Considering the new 40 Ar/ 39 Ar age determinations and field observations, the old stratigraphic division based on the lithological mapping by Zbyszewski et al. [1973, 1975] has to be partly revised. We propose the following evolution for Madeira Shield Stage (> Ma) Early Madeira Rift Phase (> Ma) [27] This volcanic phase includes the oldest subaerial exposed rocks of Madeira (basal unit). A concentration of volcanic vents and east-west oriented dike swarms in the center of the eastern half of the island suggest that volcanism during the early Madeira rift phase primarily originated from an E W orientated rift system. The oldest radiometric ages (4.63 ± 0.10 Ma) from Madeira come from an east-west oriented tholeiitic dike swarm at Porto da Cruz. Rift zones are a common feature of oceanic volcanic islands [e.g., Carracedo, 1994]. In the ideal case, triple-armed rift zones with regular geometry at 120 to one another occur as a result of least effort fracturing produced by magma-induced vertical upward loading [Luongo et al., 1991]. As is the case with many Hawaiian volcanoes (e.g., Kilauea), a third rift zone on Madeira with an angle at about 120 to both other arms could probably not develop to the NE because of the buttressing effect of the large submarine cone of Porto Santo Island (Figure 2) Desertas Rift Phase ( Ma) [28] The subaerial part of the Desertas Ridge formed between 3.2 and 3.6 Ma, during which time Madeira was almost completely inactive. At the Paul da Serra profile in central Madeira, an unconformity marks this time interval (Figure 4). In eastern Madeira north of Funchal, an unconformity also occurs separating the early and late Madeira rift phases. The lowermost flow of the late rift phase, just above the unconformity, was dated at 3.05 Ma [Watkins and Abdel-Monem, 1971]. No age data are available from rocks beneath the unconformity. A lava flow just above a prominent unconformity with the early Madeira rift phase (basal unit) in Curral das Freiras produced an age of 2.97 Ma [Mata et al., 1995]. The youngest age from below the unconformity is 3.91 ± 0.08 Ma (MA 227). These observations suggest that most of the magma supply shifted to the Desertas rift arm between 3.0 and 3.9 Ma. Although speculative, one possible cause for the shift in magma supply between 3.9 and 3.6 Ma to the Desertas rift arm may have been collapse of the NE sector of the Madeira Rift Arm (see Figure 2). The bathymetry north of Porto da Cruz is very irregular at depths between 1000 and 2000 m, possibly reflecting the presence of landslide deposits. More detailed bathymetry, however, is necessary to test this hypothesis. A similar scenario has been proposed for the shift of volcanic activity on La Palma (Canary Islands) to the N S oriented La Cumbre Vieja rift from the Taburiente shield volcano 700,000 years ago [Ancochea et al., 1994; Klügel et al., 2000]. At around 3 Ma, volcanism along the Desertas Rift arm ceased, possibly reflecting a shift in the magma supply further to the SW as a result of NE directed plate motion Late Madeira Rift Phase (3 0.7 Ma) [29] Beginning at 3 Ma, volcanic activity shifted back to Madeira. As noted above, at Paul da Serra, Curral das Freiras and Ribeiro Frio, the oldest flows overlying major unconformities separating the early and late Madeira rift phase yielded ages between 2.8 and 3.0 Ma [Watkins and Abdel- Monem, 1971; Mata et al., 1995; this study]. Field observations and age dates from thick lava sequences that cover most of the island show that the late Madeira rift phase continued without a significant pause until 0.7 Ma. Although previously mapped as belonging to the basal unit (b1ofzbyszewski et al. [1975]), scoria cone complexes such as Pico Ruivo (2.6 Ma), the highest peak on Madeira, and those near the São Lourenço peninsula ( Ma [Mata et al., 1995]) yield ages within the range for the late rift stage. These eruption centers are located along the axis of the late Madeira rift and are cut by dense dike swarms oriented in the E W direction. Four of these dikes near Pico Ruivo yield ages between Ma [Féraud et al., 1981], showing that they are contemporaneous with the thick lava sequences dipping gently to the north and south and thus probably represent feeder dikes for the late Madeira rift phase lava sequences. 19 of 26

20 Posterosional Stage (<0.7 Ma) [30] Because of the uncertainties in radiometric age dating of young magmatic rocks, the exact gap between the end of the Madeira rift phase and the posterosional stage is uncertain. The two deposits from this stage that have been dated thus far yield ages of 0.2 Ma (MA 85) and years B.P. (KIA 685 and KIA 686). It is conspicuous that the youngest activity (e.g., cinder cones in upper São Vicente valley, intracanyon flows at Porto do Moniz, Seixal, São Vicente, and the tephra layer on top of Paul da Serra) are located in the western part of Madeira, whereas the oldest rocks are found in the eastern part and the center of the island. The generally westward migration of volcanism on Madeira could potentially reflect NE motion of the African Plate Eruption Rates [31] Estimating the subaerial volume of Madeira (430 km 3 ) and the Desertas Islands (7km 3 ) and considering the oldest available radiometric age (4.63 Ma), an average magma eruption rate of 95 km 3 /Ma can be calculated for the subearial part of the shield stage. Upon closer examination of eruption rates for the different units, distinct differences become apparent. Whereas the early Madeira rift phase shows relatively high rates of 150 km 3 / Ma, magma eruption decreased during the Desertas rift phase to 20 km3/ma and increased again to 100 km 3 /Ma during the late Madeira rift phase. Although erosion may have removed a considerable volume from the Desertas, it is unlikely that eruption rates during the subaerial Desertas rift phase approached those of the Madeira rift phases. As has been suggested to explain the shift of activity to the Desertas Rift, the decrease in eruption rates between 3.0 and 3.9 Ma could also result from blocking of the magma plumbing system due to sector collapse. This may have caused an increase in intrusion relative to extrusion during the Desertas rift phase. The eruption rate during the posterosional stage of 2 km 3 /Ma is negligible. [32] The hotspots with the lowest estimated eruption rates occur in the Atlantic Ocean. These include St. Helena (24 km 3 /Ma), Bouvet (40 km 3 / Ma), Cape Verdes (40 km 3 /Ma), Ascension (60 km 3 /Ma), and Gough (110 km 3 /Ma) [Gerlach, 1990; see also Crisp, 1984; Bohrson et al., 1996]. The eruption rates reported for these islands fall within the range determined for Madeira: km 3 /Ma. It should be noted, however, that it is difficult to directly compare eruption rates for individual islands, since these estimates are often averages for the entire island and often it has not been determined which evolutionary stage(s) the island was in during its subaerial history. Eruption rates for Madeira, on the other hand, are considerably lower than those estimated for the Canary Islands (e.g., ,000 km 3 /Ma for the subaerial Miocene shield stage and up to 500 km 3 /Ma for the posterosional stage on Gran Canaria [Bogaard et al., 1988; Hoernle and Schmincke, 1993a; Schmincke and Sumita, 1998]). Madeira also shows low (subaerial) eruption rates when compared to ocean islands in the Indian and Pacific Oceans, for example, Reunion (2400 km 3 /Ma) or Mangaia ( km 3 /Ma) [Gerlach, 1990]. [33] The submarine base of the Madeira/Desertas group makes up 98% of the volcanic complex, having a volume of 26,800 km 3. The submarine volumes were estimated on the basis of the break in the bathymetry between the volcanic edifice and the gentle seafloor. Calculating an eruption rate for the submarine base is difficult owing to lack of age data and speculative assessment of the intrusive to extrusive ratio. We realize that the volume at the edifice hidden in the clastic apron may be 2 3 times more than the estimate based on the bathymetry alone, as seen, e.g., for Gran Canaria [Schmincke and Sumita, 1998]. Assuming that the Madeira/Desertas complex began forming 9.5 Myr ago (an age intermediate between the oldest ages obtained from Madeira and Porto Santo), the average rate of growth during the submarine stage was 5500 km 3 /Ma or 36 times as high as during the early subaerial rift stage on Madeira. However, in comparison to other ocean island volcanoes, Madeira has a relatively low average submarine growth rate. Production rates of >20,000 km 3 /Ma have been estimated for the nearby Canary Islands [Schmincke and Sumita, 1998] Geochronological Evolution of Porto Santo ( Ma) [34] The 40 Ar/ 39 Ar age data (samples K 49 and K 46) from the trachytic to basaltic submarine sequence in the northeast are in good agreement with paleontological data from intercalated shallow water limestones [Cachao et al., 1998]. Both types of data yield an age of 14 Ma for the end of the seamount stage in NE Porto Santo. An alkali basaltic lava flow (K 55) overlying pillow basalts but underlying a conglomerate consisting of beach 20 of 26